Various processes and apparatuses for producing urea granules have been disclosed, for example, by Japanese Patent Application Publication No. 28-4763 (1953), Japanese Patent Application Publication No. 30-6263 (1955) and Japanese Patent Application Publication No. 34-5718 (1959). In these processes and apparatuses, a urea aqueous solution or a urea melt is dropped down through a nozzle located at the top of a vertical cylindrical or prismatic granulating column. While the drops of the urea solution or melt fall down through the granulating column, the drops are dried or solidified. This type of process is referred to as an atomize-granulating process. In this type of process, the granulating column is required to be very tall so as to ensure the completion of drying or solidifying of the urea solution or melt drops. Also, this type of process is disadvantageous in that the size of the resultant granules is limited to ranges of from 1 to 2 mm. That is, it is difficult to produce urea granules having a size larger than 2 mm by the above-mentioned atomize-granulating process. Other types of atomize-granulating processes and apparatuses were disclosed, for example, in Japanese Patent Application Publication No. 34-5718 (1959), Japanese Patent Application Publication No. 39-24862 (1964) and U.S. Pat. No. 3,450,804. However, even with these types of processes and apparatuses it is still difficult to produce urea granules having a size larger than 2 mm.
Recently, the bulk blending of different fertilizer granules, such as urea, ammonium phosphate and potassium chloride, has become an important operation in the fertilizer industry. The purpose of the bulk blending is to produce a mixed fertilizer of a diesired N-P-K formulation that can be stored, shipped and spread in available commercial equipment without excessive segregation of the various components. A number of studies have been made on the segregation experienced in the blending and handling of these fertilizers. When granules having different size ranges are blended, segregation is likely to occur, resulting in incorrect composition at the point of use. The greatest single factor in producing segregation is the size distribution of the different materials. Differences in shape or density have little effect.
Accordingly, for the purpose of even distribution of the bulk blend fertilizer, it is desirable that the size of the urea granules be similar to those of the ammonium phosphate granules. However, the size of ordinary ammonia phosphate granules is in ranges of from 1.1 to 3.36 mm. That is, the size of the ammonium phosphate granules is larger than those of the urea granules produced by the conventional atomize-granulating process and apparatus.
Sometimes, the urea granules are coated with a coating material, such as sulphur and a polyolefin, in order to prepare a slow releasing nitrogen fertilizer. It appears that granules, regardless of their size, must have coatings of about the same thickness to produce similar dissolution rates. If this is a valid observation, then the amount of sulphur a substrate urea requires per unit of weight varies directly with the surface area of the urea substrate or inversely with the square of the average diameter of the granules, provided all other variables are constant. Therefore as long as agronomic benefits and handling characteristics are equal, if the size of the urea particles to be coated increases, the coating needed will decrease, reducing the production costs per unit of nitrogen and increasing the nitrogen content in the final products.
A process suitable for producing the urea granules having a size large enough for being blended with the conventional ammonium phosphate granules was disclosed in Chemical Engineering Progress, Vol. 69(2), pages 62 through 66, 1973. This type of process is referred to as "a spherodizer granulation process". In this process, seed solid particles of urea are recycled through a rotating drum, and a urea melt is atomized into the drum so that the atomized urea melt particles adhere to the urea seed particles and the adhered melt is solidified. However, this process is disadvantageous in that since the urea melt is maintained in the melt state for a long period of time, not only does the content of biuret in the resultant urea granules become undesirably high, but also, a large amount of urea melt adheres to the inside surface of the rotating drum.
Another granulating processes and apparatuses which are suitable for ammonium nitrate and potassium chloride were disclosed for example, by British Pat. No. 962,265, in which a spouted bed is used. In the process of this British patent, a vertical cylindrical vessel having a funnel-shaped bottom is employed. The lowest end of the funnel-shaped bottom is connected to a conduit for blowing a gas upward into the vessel and a thin pipe for spray-injecting a liquid containing a material to be granulated is inserted into the conduit. The top end of the liquid spray-injecting pipe is located in the lowest end of the funnel-shaped bottom of the vessel. When the liquid is spray-injected into the vessel and the gas is blown into the vessel, a spouted bed of the spray-injected liquid droplets and seed particles of the material to be granulated which have been fed thereinto is formed. However, this process is disadvantageous in that the circulation of the spray-injected liquid droplets and the seed particles in the vessel is uneven. Accordingly, the particles located close to the inside wall of the vessel and the bottom cannot be vigorously fluidized and merely form a moving bed. In this moving bed, it is impossible to produce granulates having a uniform size. Sometimes, the liquid droplets and seed particles in the moving bed adhere to each other to form large agglomerations. Also, this British patent contains no disclosure of particular conditions for producing urea granules.
Still other granulating processes and apparatuses for urea and other chemical fertilizers were disclosed, for example, in Japanese Patent Application Publication No. 46-6403 (1971), Japanese Patent Application Publication No. 47-7442 (1972), British Pat. No. 1,142,046 and U.S. Pat. No. 3,856,441, in each of which a fluidized bed is utilized. In these types of processes, a vertical cylindrical vessel, which is partitioned into an upper compartment and a lower compartment by a funnel-shaped partition converging downward to a bottom thereof, is used for forming a fluidized bed therein. The funnel-shaped partition has a number of holes formed therein.
In order to form the fluidized bed, powdery solid urea is fed into the upper compartment, a number of streams of an inert gas are blown upward into the upper compartment through the holes of the funnel-shaped partitions and a stream of an inert gas is blown vertically upward into the upper compartment through a center of the bottom of the funnel-shaped partition. A liquid containing a material to be incorporated onto the powdery urea fine particles is spray-injected into a middle or upper portion of the fluidized bed. The spray-injected liquid droplets adhere to the powdery urea fine particles and the adhere liquid is dried in the fluidized bed. In the abovementioned process, the circulation, of the urea fine particles and the spray-injected liquid droplets in the fluidized bed can be made more uniform than in the above-mentioned spouted bed process. However, it was found that, in the above-mentioned fluidized bed processes, since the feed of the liquid containing the material to be incorporated onto the powdery urea fine particles is carried out by using a nozzle projected through a side wall of the upper compartment of the cylindrical vessel into the middle or upper portion of the upper compartment, the fed liquid adheres to the surface and top end of the nozzle and the adhered liquid layer is dried on the nozzle. Also, the powdery urea fine particles adhere to the liquid layer so as to form a thick layer or large lumps on the nozzle. This phenomenon promotes the formation of large agglomerations of the urea granules and causes the efficiency of producing the urea granules to be reduced.
Also, in the above-mentioned fluidized bed processes, it was found that the powdery urea fine particles to be fed into the fluidized bed have too small of a size as seeds for producing the urea granules having a relatively large size. According, not only is the efficiency of producing the urea granules too low, but also the resultant small urea granules tend to adhere to each other so as to form large agglomerations. Furthermore, in the conventional fluidized bed processes, it was found that since the resultant urea granules are discharged through the bottom of the funnel-shaped partition into a collecting vessel located below the lower compartment of the cylindrical vessel, the fluctuations in flow rate, flow velocity and pressure of the inert gases blown into the upper compartment during the urea granule producing process, results in a change in the stability of the fluidized particles. Sometimes, a large amount of the fluidized particles having various sizes fall down directly into the collecting vessel. The particles received in the collecting vessel can not be recycled into the fluidized bed. Accordingly, it is difficult to selectively discharge the resultant urea granules having a desired size. This difficulty results in unevenness in size of the discharged urea granules.
Under these circumstances, an improved process and apparatus for producing urea granules having a uniform size and quality without difficulty are desired.